The Justice League of America has a flag, at least in
the 2008 movie "Justice League: The New Frontier": Blue with its logo.
The movie is set in the late 50's, so it may be an older version.Eugene Ipavec, 7 June 2009

In the graphic novel JLA: earth 2, this flag represent the "United States of Amerika"
(note the "K"), a version of the USA from an evil mirror universe. (But then, aren't they all?)

The image is similar the real US national flag with the canton in
red, the red bars in blue and the white bits in black,
the bars are also arranged verticaly instead of
horizontaly. the number of stars and bars is mostly a
guess since the flags are shown folded but there seem
to be more bars than in real life.Marc Pasquin, 10 April 2005